There is a method known in the related art whereby noise is simply removed from an image in the real space, expressed in a single channel band (see patent reference 1). In this method, a high-frequency component signal containing an edge component is extracted through high-pass filter processing executed by using a fixed filter coefficient and the noise component contained in the high-frequency component signal is estimated through nonlinear conversion processing. Adaptive filtering such as a filter processing, which assures higher resolution and has a heavy processing load by using a filter coefficient that changes in correspondence to the image structure, is not executed in this method. However, there is an issue yet to be addressed in the method in that the high-pass-filter size cannot easily be increased without compromising the simplicity of the processing, making it difficult to effectively remove noise occurring over a long cycle.
There is also a method known in the related art whereby noise is removed by using multiple channel band image expressed based upon multi-resolution representation. This method may be realized most simply in conjunction with the coring technology whereby the slight fluctuation component in a high-pass sub-band images generated through multi-resolution transformation is set to zero. Patent reference 2 discloses a nonlinear coring technology through which the coring operation is executed so as to achieve a gradual characteristics curve instead of through threshold value processing.
While patent reference 1, too, describes an example in which the real space processing is adopted in conjunction with multi-resolution representation, the technology disclosed in patent reference 1 is basically equivalent to a high-pass sub-band nonlinear coring processing technology. Namely, it teaches a technology whereby the operation for generating a high-pass component in the real space processing is replaced by high-pass sub-band image generation achieved through the multi-resolution transformation, which ultimately is simply an example of a variation of processing for coring the high-pass sub-band images.
In the high-pass sub-band coring processing executed on the multi-resolution images, noise is removed simply based upon the value of the sub-band coefficient itself. Since this noise removal processing is not by any means high-definition noise removal processing, the resulting image quality is hardly ideal. This issue is addressed in patent reference 3 and patent reference 4, each disclosing a technology whereby noise removal processing is executed by correcting the sub-band coefficient based upon the high-pass sub-band coefficient and nearby sub-band coefficients instead of using the high-pass sub-band coefficient alone so as to improve the image quality.
Patent reference 1: Japanese Laid Open Patent Publication No. 2002-74356
Patent reference 2: U.S. Pat. No. 5,467,404
Patent reference 3: U.S. Pat. No. 5,461,655
Patent reference 4: U.S. Pat. No. 5,526,446